There have been active researches directed to separating only specific polarized light from ordinary light, and various forms of polarizing elements have been developed to this end. Among these polarizing elements are, for example, birefringent polarizing elements that use birefringent (optically anisotropic) crystals such as calcite; dichroic polarizing elements in which a dichroic pigment or an organic pigment (dye) such as iodine is orientationally dispersed in a polymer; and reflective polarizing elements of the property to reflect polarized light of one direction with the use of an element such as a multilayered film of a controlled refractive index. The dichroic polarizing element, with its high dichroic properties, enables control of transmitted light, and therefore has been suitably used for liquid crystal display devices, one application of the polarizing element.
However, while the dichroic polarizing element transmits 50% of all incident light (maximum transmittance of 46% by excluding 4% surface reflection), it absorbs a component having an azimuth perpendicular to the transmitted light. Because 50% of the light is lost, the dichroic polarizing element is not satisfactory in terms of efficient use of light.
To avoid such absorption of light in the dichroic polarizing element and improve the efficiency of using light, attempts have been made to use the dichroic polarizing element in combination with a reflective polarizing element. This method attempts to improve the efficiency of using light by retroreflecting the light perpendicular to the transmission axis and absorbed by the polarizing element.
As used herein, the “reflective polarizing element” basically refers to an element that utilizes the optical reflection and interference properties, and that takes advantage of the reflecting property to separate polarized light that would otherwise be lost by being absorbed by the dichroic polarizing plate.
A known example is a reflective polarizing element that uses a cholesteric liquid crystal layer and a ¼ wavelength plate in combination (see Patent Document 1). In the reflective polarizing element described in Patent Document 1, the cholesteric liquid crystal layer has the property to transmit right- (or left-) circularly polarized light, and reflect left- (right-) circularly polarized light of a wavelength corresponding to its spiral pitch. The ¼ wavelength plate then converts the transmitted circularly polarized light to linearly polarized light, selectively producing linearly polarized light.
Further, Patent Document 2 describes a reflective polarizing element that utilizes the interference of a birefringent multilayered film. In the reflective polarizing element of Patent Document 2, the polarized light is separated by an oriented multilayered film of two kinds of polymer films made of birefringent material. The oriented multilayered film is commercially available from 3M under the trade name D-BEF (brightness enhancement film) series.
Patent Document 3 proposes a method of polarization separation, sharing the same principle as the method of Patent Document 2, in which a continuous phase and a discontinuous phase are created with the use of a simple polymer blend.
Further, Patent Document 4 proposes a polarizing element that uses a birefringent material and prisms in combination. The polarizing element described in Patent Document 4 is structured to include prisms on a surface of a birefringent polymer substrate. The prism angle is set so that one linearly polarized light component is reflected at the prism with an angle smaller than the critical angle, and that the other linearly polarized light component is reflected at the prism with an angle equal to or greater than the critical angle, taking advantage of a difference in refractive index between the cross sectional direction and the lengthwise direction of the prisms. With this setting, total reflection occurs for the light equal to or greater than the critical angle, reflecting it back to the incident light side, and thereby separating the polarized light.    [Patent Document 1] JP-A-8-271731    [Patent Document 2] U.S. Pat. No. 3,610,729    [Patent Document 3] JP-T-2000-506994 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application)    [Patent Document 4] JP-A-2006-220879